Alkylated aryl phosphates are known in the art to be useful as flame-retardants. These compounds can be formed by a number of methods commonly used in the art. For example, it is known to prepare mixed synthetic triaryl phosphates by alkylating phenol with alkenes such as propylene or isobutylene to obtain a mixture of phenol and substituted phenols. According to U.S. Pat. No. 4,093,680 this alkylate mixture is then reacted with phosphorus oxychloride (POCl3) to form a mixed triaryl phosphate ester. The product mix is a statistical mixture based on the composition of the starting alkylates and always includes some fraction of triphenyl phosphates (“TPP”), usually from 5 to 50 percent. The product's physical properties are determined by the degree of alkylation of the phenol. A highly alkylated phenol mixture will result in a more viscous phosphate ester product than one less substituted.
Triphenyl phosphate is a by-product of the alkylated phenyl phosphate formation reaction and is unwanted in the final product because of environmental concerns. For example, TPP has been classified as a marine pollutant in some jurisdictions. Thus, there has been much attention in the art given to removing TPP from alkylated phenyl phosphates. For example, U.S. Pat. No. 5,206,404 discloses that wipe film purification can be used to produce mixed alkylated triphenyl phosphates with TPP concentrations of less than 2 wt %. The '404 patent also discloses that an undesirable method of reducing the TPP concentration of alkylated phenyl phosphates is by fractional distillation.
U.S. Pat. No. 6,232,485 discloses a process for producing a liquid triaryl phosphate ester having low triphenyl phosphate concentrations and low viscosity comprising (a) an alkylation stage wherein a phenol is reacted with an olefin having 2 to 12 carbon atoms in the presence of a strong acid catalyst to give a reaction product comprising a mixture of meta and para alkylated phenols; (b) a transalkylation stage wherein the mixture of alkylated phenols from the alkylation stage is heated in the presence of a strong acid catalyst to increase the meta isomer content of the mixture to at least 20% whilst maintaining a phenol level below 22%; and (c) a phosphorylation stage. In the '485 process, the mixture of alkylated phenols from the transalkylation stage is reacted with a phosphorylating agent, and the strong acid catalyst used in stages (a) and (b) is a Bronsted acid having an acid strength of less than zero.
However, none of the prior art processes are without drawbacks. For example, the '404 patent teaches that repeated passes through the wipe film evaporator may be necessary to reduce TPP concentrations to desired levels. Fractional distillation has also been taught as a method to reduce TPP concentrations in the final triaryl phosphate ester product, which also has drawbacks. As noted in the '404 patent, fractional distillation of the final triaryl phosphate ester product produces a product that has undesirable color and acidity levels. Likewise, the '485 patent, and others such as U.S. Pat. Nos. 4,069,279, 4,139,487 and 6,232,485 disclose processes that produce products comprising mixtures of alkylated phenyl phosphates with alkyl groups distributed on more than one phenyl group. These product distributions have undesirable properties such as high viscosity, inconsistent viscosity characteristics, and high triphenyl phosphate concentrations.
Thus, there exists a need in the art for low triphenyl phosphate, high phosphorous alkylated phenyl phosphates with high ortho alkylation, and a process for their formation.